The present invention relates in general to a suspension system for a steerable wheel of a vehicle and more particularly to an improvement in a lower double-joint front suspension system for vehicles.
A lower double-joint front suspension system is well known in the art and disclosed in Japanese Patent Provisional Publication No. 8-156545. The lower double-joint front suspension system includes a steering knuckle or wheel carrier rotatably carrying thereon a front wheel, an upper suspension link and a pair of lower suspension links. The upper suspension link is connected to an upper connecting portion of the wheel carrier. The lower suspension links are connected to lower connecting portions of the wheel carrier, which are spaced apart in the longitudinal direction of the vehicle. By the upper and lower suspension links, the wheel carrier is supported in a way as to be steerable or turnable.
In this kind of suspension system, the front wheel which serves as an inner wheel during cornering encounters the following problem which will be described with reference to FIG. 6.
FIG. 6 is a view for illustrating an operation of a lower double-joint front suspension system for a left front wheel 1 when the left front wheel is steered to the left, i.e., steered so as to move from a neutral position (i.e., position for straight ahead driving) indicated by the solid line to a steered or turned position indicated by a dotted line. By such movement of the left front wheel 1, a transverse link 2, which constitutes front one of the above described lower arms, is caused to pivot about a vehicle body side connecting end portion 2a from a position indicated by a solid line to a position indicated by a dotted line, thus causing a wheel side connecting end portion 2b to move into the position 2bxe2x80x2. 
On the other hand, a compression rod 3, which constitutes rear one of the above described lower arms, pivots about a vehicle body side connecting end portion 3a to cause a wheel side connecting end portion 3b to move into the position 3bxe2x80x2. 
Accordingly, the center of turning of the left front wheel 1 which serves as an inner wheel at this turning of the vehicle to the left, is moved from the point 4, where a straight line joining the center of the vehicle body side connecting end portion 2a and the center of the wheel side connecting end portion 2b of the transverse link 2 intersects a straight line joining the center of the vehicle body side connecting end portion 3a and the center of the wheel side connecting end portion 3b of the compression rod 3, to the point 5, where a straight line joining the center of the vehicle body side connecting end portion 2a and the center of the wheel side connecting end portion 2bxe2x80x2of the turned transverse link 2 intersect a straight line joining the center of the vehicle body side connecting end portion 3a and the center of the wheel side connecting end portion 3bxe2x80x2of the turned compression rod 3. Thus, the suspension system has a positive scrub radius L1, which is the distance from the turning center 5 to the center of tire contact when observed in a plan.
During such turning or cornering of the vehicle, a braking force FG applied to the center of tire contact is transmitted to the vehicle body side connecting end portion 3a of the compression rod 3 and causes a resilient bushing constituting part of the vehicle body side connecting end portion 3a to deform resiliently and therefore the vehicle body side connecting end portion 3a to move rearward and inboard as indicated by the arrow xcex1 due to its resilient deformation characteristics.
Although the resilient deformation of the vehicle body side connecting end portion 3a of the compression rod 3 in the direction indicated by the arrow xcex1 does not cause any substantial change of the positive scrub radius L1, the braking force FG tends to cause the steered or turned left front wheel 1 to further turn about the turning center 5 to the left due to the positive scrub radius L1, thus causing a problem of deteriorating the returnability of the steering wheel to its neutral position (i.e., position for straight-ahead driving).
It is accordingly an object of the present invention to provide a suspension system for a steerable wheel which can improve the returnability of a steering wheel to its neutral position.
It is a further object of the present invention to provide a suspension system of the foregoing character which can reduce a positive scrub radius for the steerable wheel when the steering wheel serves as an inner wheel and is subjected to a braking force, whereby to reduce the moment of the force caused by the braking force and tending to turn the inner steerable wheel further in the steered direction.
To accomplish the above objects, the present invention provides a suspension system for a steerable wheel which comprises a wheel carrier rotatably carrying thereon the steerable wheel and having a pair of lower connecting portions spaced apart in the longitudinal direction of a vehicle body, and a pair of lower suspension links pivotally connected to the lower connecting portions of the wheel carrier for supporting the wheel carrier upon the vehicle body in such a manner as to allow the wheel carrier to be steerable. One of the lower suspension links which is disposed in the rear of the other has a connecting end portion at which it is connected to the vehicle body. The connecting end portion of the aforementioned one lower suspension link has a first rigidity to resist change in form in the longitudinal direction of the vehicle body and a second rigidity to resist change in form in the width direction of the vehicle body, the second rigidity being smaller than the first rigidity.